This application claims the benefit of Korean Patent Application No. 1999-67854, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to X-ray image sensors. More particularly, it relates to X-ray image sensors having a TFT (Thin Film Transistor) array, and to a method for fabricating the same.
2. Discussion of the Related Art
X-ray detection has been widely used for medical diagnosis. X-ray detection typically uses an X-ray film to produce a photograph. Therefore, some predetermined developing and printing procedures are required to produce the photograph.
However, digital X-ray image sensors that employ TFTs (Thin Film Transistors) have been developed. Such X-ray image sensors have the advantage that a real time diagnosis can be obtained.
FIG. 1 is a schematic, cross-sectional view illustrating the structure and operation of an X-ray image sensing device 100. Included are a lower substrate 1, a thin film transistor 3, a storage capacitor 10, a pixel electrode 12, a photoconductive film 2, a protection film 20, a conductive electrode 24 and a high voltage D.C. (direct current) power supply 26.
The photoconductive film 2 produces electron-hole pairs 6 in proportion to the strength of external signals (such as incident electromagnetic waves or magnetic waves). That is, the photoconductive film 2 acts as a converter that converts external signals, particularly X-rays, into electric signals. Either the electrons or the holes are then gathered by the pixel electrode 12 as electric charges. The pixel electrode 12 is located beneath the photoconductive film 2. Which electric charges that is gathered depends on the voltage (Ev) polarity that is applied to the conductive electrode 24 by the high voltage D.C. power supply 26. The gathered electric charges accumulate in the storage capacitor 10, which is formed in connection with a grounding line. Charges in the storage capacitor 10 are then selectively transferred through the TFT 3, which is controlled externally, to an external image display device that forms an X-ray image.
In such an X-ray image sensing device, to detect and convert weak X-ray signals into electric charges it is beneficial to decrease the trap state density (for the electric charge) in the photoconductive film 2, and to decrease charge flow in non-vertical directions. Decreasing non-vertical charge flow is usually accomplished by applying a relatively high voltage between the conductive electrode 24 and the pixel electrode 12.
Electric charges in the photoconductive film 2 are trapped and gathered not only on the pixel electrode 12, but also over the channel region of the TFT 3. Even during the OFF state, the electric charges trapped and gathered on the pixel electrode 12 and on the channel region of the TFT 3 induce a potential difference between the TFT 3 and the pixel electrode. This has a similar effect as the TFT 3 being in the ON state. This adversely affects the switching of the TFT 3 and increases the OFF state leakage current. Such can result in an undesired image.
FIG. 2 is a plan view illustrating a pixel of the X-ray image sensor panel. Shown are the TFT 3, a storage capacitor xe2x80x9cSxe2x80x9d and gate and data lines 30 and 40.
The gate line 30 is arranged in one direction and the data line 40 is arranged perpendicular to the gate line 30. The TFT 3 is formed near the crossing of the gate and data lines 30 and 40. The TFT 3 includes a gate electrode 32, which is formed by an elongation of the gate line 30, and a source electrode 42, which is formed by an elongation of a data line 40. The TFT 3 also includes a drain electrode 44 that is spaced apart from the source electrode 42.
A ground line 52 is parallel to the data line 40 and perpendicular to the gate line 30. The ground line 52 crosses the storage capacitor area and acts as a common electrode that is shared by adjacent pixels. A ground line contact hole 54 is formed over the ground line 52 such that a capacitor electrode 46 contacts the ground line 52 through the ground line contact hole 54. Two or more two ground line contact holes can be formed over the ground line 52.
The storage capacitor xe2x80x9cSxe2x80x9d, which stores the electric charges, is comprised of the capacitor electrode 46, a pixel electrode 56, and a dielectric layer (not shown) that is interposed between the capacitor electrode 46 and the pixel electrode 56. The pixel electrode 56 extends over the TFT 3 and acts as the other capacitor electrode. In order to couple the electrons (which come from the TFT xe2x80x9c3xe2x80x9d) with the holes (which are stored in the storage capacitor xe2x80x9cSxe2x80x9d), the pixel electrode 56 is electrically connected to the drain electrode 44 via a drain contact hole 50 and via an auxiliary drain electrode 48.
A gate pad 34 is formed at one end of the gate line 30, and a data pad 41 is formed at one end of the data line 40. The data pad 41 includes a data pad connector 45 that contacts the data line 40 through the first data pad contact hole 43. Thus, the data line 40 is electrically connected to the data pad 41.
The principle and the function of the X-ray image sensing device will now be explained.
The holes (electric charges) generated in a photoconductive film (not shown) are accumulated on the pixel electrode 56 and stored in the storage capacitor xe2x80x9cSxe2x80x9d with the capacitor electrode 46.
The holes in the storage capacitor xe2x80x9cSxe2x80x9d are transferred to the source electrode 42 through the drain electrode 44 when the TFT 3 is turned ON. The holes arrive at an external image display device that forms an X-ray image. At this time, the ground line 52 removes the residual charges (holes) that are not transferred to the external image display device, i.e., that remain in the storage capacitor xe2x80x9cSxe2x80x9d. Of course, the foregoing discussion of holes is to be taken in an engineering context as holes are not physical currents.
FIGS. 3A to 3E are cross-sectional views, taken along line IIIxe2x80x94III of FIG. 2, that illustrate manufacturing processes of an X-ray image sensor panel.
Referring to FIG. 3A, a gate electrode 32, a data pad 41 and a data pad connector 45 are formed on a substrate 1 by depositing and patterning a low resistant metallic material such as Aluminum (Al) or Al-alloy (for example, AlNd) using a first mask. The substrate 1 is made of a glass substrate, which is mainly used when processing is performed at a low temperature, or of a quartz glass, which has a high melting temperature and is more suitable for high temperature processing.
FIG. 3B illustrates a manufacturing step of forming a first insulation layer 60 and semiconductor layers 65 and 63. The first insulation layer 60 is formed at a thickness of about 4000 xc3x85 by depositing an inorganic insulation material such as Silicon Nitride (SiNx) or Silicon Oxide (SiOx). Silicon Nitride (SiNx) is beneficially used in a related art.
After that, the semiconductor layers are formed by depositing a pure amorphous silicon 62 and a doped amorphous silicon 64 in sequence. The CVD (Chemical Vapor Deposition) or the Ion Injection Method are beneficially used to form the doped amorphous silicon layer 64. The CVD method is employed in a related art.
The semiconductor layer 65 and the island-shaped semiconductor layer 63 are formed by patterning the pure amorphous silicon and the doped amorphous silicon using a second mask. The island-shaped semiconductor layer 63 acts as an auxiliary electrode of a ground line that will be formed later.
Referring to FIG. 3C, a first data pad contact hole 43 is formed over the data pad connector 45 by patterning the first insulation layer 60 using a third mask. Then, a data line 40, a source electrode 42, a drain electrode 44 and a ground line 52 are formed by depositing and patterning a second metal, such as Chrome (Ch) or a Cr-alloy, using a fourth mask. The data line 40 is formed such that it is electrically connected to the data pad 41 through the first data pad contact hole 43. A portion of the doped amorphous silicon layer 64 on the pure amorphous silicon layer 62 is then etched, using the source and drain electrodes 42 and 44 as masks, to form a channel region xe2x80x9cCHxe2x80x9d. Thus, the TFT 3 (see FIG. 2) is completed.
As shown in FIG. 3D, a second insulation layer 66 is formed over the TFT, over the ground line 52 and on the first insulation layer 60. A first drain contact hole 50a is then formed to expose a portion of the drain electrode 44, and a ground line contact hole 54 is formed to expose a portion of the ground line 52, by use of a fifth mask. After that, an auxiliary drain electrode 48 and a capacitor electrode 46 are formed by depositing and patterning a transparent conductive material using a sixth mask. The auxiliary drain electrode 48 contacts the drain electrode 44 via the first drain contact hole 50a, and the capacitor electrode 46 contacts the ground electrode 52 via the ground line contact hole 54. The auxiliary drain electrode 48 and the capacitor electrode 46 are spaced apart from each other.
Referring to FIG. 3E, a third insulation layer 68 is formed on the second insulation layer 66, on the auxiliary drain electrode 48, and on the capacitor electrode 46. A second drain contact hole 50b is then formed to expose a portion of the auxiliary drain electrode 48 by patterning the third insulation layer 68 using a seventh mask. After that, a transparent conductive material is deposited and patterned to form a pixel electrode 56 using an eighth mask. The pixel electrode 56 is electrically connected to the auxiliary drain electrode 48.
Finally, a second data pad contact hole 47 is formed to expose the data pad 41 by patterning the first, second and third insulation layers 60, 66, and 68 using a ninth mask.
Therefore, as described above, the conventional X-ray image sensor is formed using a nine-mask process.
Although not depicted, the next step is the application of a photoconductive film. That material converts received external signals (X-rays) into electric charges. The photoconductive film is beneficially comprised of an amorphous selenium compound that is deposited with a thickness of 100 to 500 xcexcm by an evaporator. However, other X-ray photoconductive films that having low dark conductivity and high sensitivity to external signals, for example HgI2, PbO2, CdTe, CdSe, Thallium bromide, or Cadmium sulfide can also be used. When the photoconductive film is exposed to X-rays, electron-hole pairs are produced in the photoconductive film in accordance with the strength of the x-rays.
After the application of the X-ray photoconductive film, a transparent conductive electrode that passes X-ray is formed. When a voltage is applied to the transparent conductive electrode while X-rays are being irradiated, electron-hole pairs formed in the photoconductive film are separated into charges that are gathered to the pixel electrode and stored in the storage capacitor xe2x80x9cSxe2x80x9d (see FIG. 2).
FIG. 4, a cross-sectional view taken along line IVxe2x80x94IV, illustrates the gate pad 34 (see FIG. 2). The gate pad 34 is extended from the gate line 30, and the gate pad contact hole 35 is formed to expose a portion of the gate pad 34 by patterning the first, second and third insulation layers 60, 66, and 68.
As described above, nine mask processes are used to fabricate the X-ray image sensor. Each mask process requires several steps, such as a cleaning step, a depositing step, a baking step, and an etching step. Therefore, if the number of mask processes is decreased by only one mask, the throughput and manufacturing yields can dramatically increase and the manufacturing costs and time can be reduced.
Accordingly, the present invention is directed to an X-ray image sensor and to a method for fabricating the same and that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an X-ray image sensor having simpler processing steps while forming a first data contact hole, a semiconductor layer, and an island-shaped semiconductor layer.
Another object of the present invention is to provide an X-ray image sensor having improved yields.
A further object of the invention is to provide a method of forming an X-ray image sensor which can reduce processing error during production by reducing mis-alignment.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the above objects, the present invention provides an X-ray image sensor, including: a substrate having a pixel region with a switching region at one corner of the pixel region; a thin film transistor (TFT) formed on the switching region of the substrate, the TFT having a gate electrode, a first insulation layer, a pure amorphous silicon layer, a doped amorphous silicon layer, and source and drain electrodes; an island-shaped first insulation layer and a island-shaped semiconductor layer formed over the substrate in the pixel region; a ground line formed on the island-shaped semiconductor layer; a second insulation layer formed on the TFT, on the substrate, and on the ground line, the second insulation layer having a first drain contact hole which exposes a portion of the drain electrode, and a ground line contact hole which exposes a portion of the ground line; an auxiliary drain electrode formed on the second insulation layer and contacting the drain electrode through the first drain contact hole; a capacitor electrode formed on the second insulation layer and contacting the ground line through the ground line contact hole; a third insulation layer formed on the second insulation layer, on the auxiliary drain electrode, and on the capacitor electrode, the third insulation layer having a second drain contact hole which exposes a portion of the auxiliary drain electrode; and a pixel electrode formed on the third insulation layer and contacting the auxiliary drain electrode through the second drain contact hole.
Beneficially, the pixel electrode extends over the source and drain electrodes.
Beneficially, the auxiliary drain electrode and the capacitor electrode are made of the transparent conductive material, and the second insulation layer is made of BCB (benzocyclobutene).
In order to achieve the above objects, the invention also provides a method for fabricating an X-ray image sensor, including: providing a substrate that has a pixel region having a switching region at one comer of the pixel region and portions defined for data and gate lines; forming a gate electrode, a data pad connector, and a data pad on the substrate by depositing and patterning a first metallic material using a first mask process; sequentially forming a first insulation layer, a pure amorphous silicon layer, and a doped amorphous silicon layer, the first insulation layer covering the substrate, the gate electrode, the data pad connector, and the data pad; forming a first data pad contact hole, a semiconductor layer, and an island-shaped semiconductor layer by patterning the doped amorphous silicon layer, the pure amorphous silicon layer, and the first insulation layer using a second mask process, the first data contact hole exposing a portion of the data pad connector; forming a data line, a source electrode, a drain electrode, and a ground line on the semiconductor layer and on the island-shaped semiconductor layer by depositing and patterning a second metallic material using a third mask process, the data line contacting the data pad connector through the first data pad contact hole; forming a second insulation layer on the TFT, on the substrate, and on the ground line; forming a first drain contact hole and a ground line contact hole by patterning the second insulation layer using a fourth mask process, the first drain contact hole exposing a portion of the drain electrode and the ground line contact hole exposing a portion of the ground line; forming an auxiliary drain electrode and a capacitor electrode on the second insulation layer by depositing and patterning a transparent conductive material using a fifth mask process, the auxiliary drain electrode contacting the drain electrode through the first drain contact hole and the capacitor electrode contacting the ground line through the ground line contact hole; forming a third insulation layer on the second insulation layer, on the auxiliary drain electrode, and on the capacitor electrode; forming a second drain contact hole to expose a portion of the auxiliary drain electrode by patterning the third insulation layer using a sixth mask process; forming a pixel electrode on the third insulation layer by depositing and patterning a transparent conductive material using a seventh mask process, the pixel electrode contacting the auxiliary drain electrode through the second drain contact hole; and forming a second data pad contact hole to expose the data pad by patterning the first, second and third insulation layers and the pure and doped amorphous silicon layers.
In order to achieve the above objects, the invention also provides a method for fabricating an X-ray image sensor, further including forming a channel region by removing a portion of the doped amorphous silicon layer over the pure amorphous silicon layer using the source and drain electrodes as masks after forming the source and drain electrodes.
Beneficially, the auxiliary drain electrode, the capacitor electrode, and the pixel electrode are formed from indium tin oxide (ITO) or from indium zinc oxide (IZO), and the gate electrode, the data pad connector, and the data pad are formed from a material selected from a group consisting of aluminum (Al) and aluminum-neodymium (AlNd).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.